Image projection systems are known to be highly susceptible to geometric image distortions due to their positional freedom. Essentially, any deviation from perpendicularity of the axis that connects the light source of a projector to a viewing surface (normally the optical axis) causes a so-called keystone distortion. Although occurring in rear-projection configurations too, this effect is more pronounced in front-projection configurations that have typically a long throw ratio.
Several prior art patents address electronic correction of geometric distortions, in addition to older mechanical solutions. A majority of the electronic correction methods exploit some form of calibration test patterns that consist of precisely positioned objects (lines, circles, rectangles, etc.) on the projection surface; or use a fixed reference such as physical bezels of the projection surface. These solutions are generally suitable for calibrations in controlled environments, where devices are stationary or have limited displacements. Also, they may require precise alignment jobs. The U.S. Pat. No. 8,406,562 teaches a comprehensive test pattern and reference based calibration techniques applicable to most image systems including rear and front view projectors, and various surface geometries. Another class of calibration methods, for example as taught in the U.S. Pat. No. 6,877,863; relies on position and orientation measurement sensors to quantify distortion parameters. That in turn requires extra components and tools.
In some set-ups, such as a car Heads-Up-Display (HUD) or a Helmet-Mounted-Display (HMD), the projectors are susceptible to constant displacements from the ideal (also referred to as reference) position. Yet, it is crucial to have an undistorted image at all time. Therefore, there is a need for fast, frequent, and automatic calibration on the run. Further, it would be desirable to eliminate additional resources (test patterns, test pattern generators) and alignment tasks needed to perform the calibration.
The present invention takes advantage of digital imaging combined with image analytic techniques to extract projectors' rotational angles from naturally occurring scenes, instead of calibration patterns. Capture devices equipped with Wide-Angle (WA) and particularly Ultra Wide-Angle (UWA) lenses, lenses having a field of view of at least 180°, almost guarantee capture of useful information from a scene. This approach would be at the expense of complexity of UWA mapping calculations and performing heavier image processing. The co-pending patent application PCTUS2012/027189 teaches techniques of UWA lens mapping; content of which will be used as a reference.